The Exponential Law of Tissue Recovery Applied to Radium and Radon Dosage

Abstract

IT has often been assumed that tissue recovery from radiation effects follows an exponential law. By the use of this law in the saturation technic of radiation therapy, tissue recovery is calculated for the time between treatments. When a treatment lasts for only a short time, the tissue recovery during the period of treatment is negligible; but when a treatment lasts for many hours, as many radium and radon treatments do, the tissue recovery during this period may not be negligible. In some instances, it seems possible that a better correlation between tissue effect and the dose given might be obtained if the saturation dose, which takes into account the tissue recovery during the treatment, was considered rather than the total dose. The expression “tissue recovery from radiation effects” is indefinite because little is known about the effects of radiation on tissue. Most of the effects usually considered to be due to irradiation are probably secondary effects caused by a primary effect. The primary effect is caused by an absorption of radiation, or, since radiation is energy, by an absorption of energy; the magnitude of this primary effect must be proportional to the amount of energy absorbed. Hence, the primary effect must be an increase of the energy content of the tissue. Tissue recovery will be considered here as a process tending toward a return to the normal energy content. The amount of energy absorbed will depend upon many factors and cannot be easily determined, but, for a specific type of radiation, it will be proportional to the amount of energy available. For any specific case it will be proper, therefore, to use a measure of the available energy as a measure of the primary effect. When treatments are given with radium or radon applicators, the strength of the applicator, in mg. of radium or mc. of radon, is a measure of the available energy. The usual notation of dosage in mg.-hr. or mc.-hr. can serve as a measure of the magnitude of the primary effect produced in the tissue, and also it can serve as a measure of the magnitude of the primary effect remaining in the tissue at any future time. The fraction of the radiation effect that recovers in unit time is believed to vary with the quality of radiation used. This fraction, which can be called the “tissue recovery constant” and which can be calculated from data given by Weatherwax (2), is 0.044 each day for radiation filtered through 1 mm. of platinum, and 0.035 each day for radiation filtered through 2 mm. of platinum. The value of the recovery constant used in computing the data presented here is 0.05 each day, which is assumed to be approximately correct for radiation filtered through 0.5 mm. of gold or platinum.